A study published in Biosensors gives an overview of silicon photomultipliers and their potential in clinical and bio-photonic applications. It also compared the silicon photomultipliers with different photodetectors based on their circuit design parameters.
What are Silicon Photomultipliers?
Silicon photomultipliers (SiPMs) are single-photon avalanche diode (SPAD) based solid-state photodetectors. They are arrays of avalanche photodiodes operating in Geiger mode, designed to detect extremely weak light down to a single photon.
Several thousands of SPADs are linked simultaneously to make a SiPM, depending on the light source and application. Each SPAD contains an integrated series resistor that quells the avalanche and resets the photodiode for the incoming photon.
A silicon photomultiplier can attain a very high signal-to-noise ratio using an internal amplification technique, making it possible to detect individual photons with exceptional temporal precision. SiPMs, in contrast to other photodetectors, generate sharp, quasi-digital pulses for each detected photon.
SiPM offers a very high gain, a quick reaction time, and a short recovery time. This makes them ideal for numerous photometry applications.
Common SiPM applications include LiDAR and 3D ranging, high-energy physics, sorting and recycling, aero particle physics, hazard and threat detection, scintillators, fluorescence spectroscopy, medical imaging, and bio-photonics.
Significance of Silicon Photomultipliers in Bio-Photonics
Bio-photonics exploits light-tissue interactions, such as scattering, reflectance, absorption, fluorescence, and time-of-flight (ToF), to determine tissue composition and properties.
These diagnostic procedures can be carried out on ex-vivo and in-vivo tissue using minimally invasive approaches, providing appealing substitutes for currently used medical diagnostic equipment and facilitating research for future medical devices.
Fiber-coupled devices offer the potential for long source-detector distances and can image deep into the body. These tools are typically catheter- or guidewire-based optical fiber devices used for surgical guiding and diagnostics.
However, fiber-coupled devices need high-quality photodetectors with a high temporal resolution and the ability to detect light energies in the picowatt or sub-picowatt range.
Designing a wide dynamic range, low-noise, and highly sensitive application-specific integrated circuit (ASIC) for silicon photomultipliers with the potential for device miniaturization, surgical tool integration, and heterogeneous integration would improve clinical utility. The combination of ASIC and SiPM will be particularly useful for auto-fluorescence-assisted margin biopsy of brain tumors.
Experimentation and Comparison of Photo Detectors
Researchers compared three bio-photonics detectors: a PIN diode, a silicon photomultiplier, and an avalanche photodetector (APD). They focused on their performance and circuitry.
Detector performance comparison
The SiPM offers various benefits over the other two detectors. The SiPM is the smallest of the three detectors while maintaining the highest ratio of the active area to fill factor. The silicon photomultiplier also has the highest timing resolution, which is ideal for rapid measurements in auto-fluorescence.
In addition, the silicon photomultiplier has the highest responsivity as it produces the maximum number of carriers from photon absorption. Therefore, the SiPM can generate a higher photocurrent than other types of detectors, resulting in improved resolution and SNR.
Readout circuit comparison
The inherent high gain of the silicon photomultiplier necessitates a lower resistance gain of the trans-impedance amplifier. This minimizes the thermal noise induced by the current-to-voltage conversion.
Tissue recognition project
Researchers employed SiPMs to develop a bio-photonics tissue-recognition biopsy device on a single-board system. The experiment proved that the photodetector had a linear response even when exposed to dim light.
Important Findings of the Study
Careful selection of the trans-impedance amplifier gain resistance, nominal overvoltage, and circuit architecture enables PCB-mounted SiPM detectors to deliver a higher dynamic range at lower optical power than PIN diode- and APD-based systems.
The SiPM enabled a reasonable bias voltage in the region of 30 V and a negligible dark current, making it ideal for applications with limited power. The SiPM's spectral response is also ideal for auto-fluorescence measurements in the visible spectrum.
Even though single photon counting is a superior measurement method in bio-photonics, single photon counting has a restricted dynamic range and is unsuitable for auto-fluorescence and diffuse reflectance spectroscopy measurements. This study, therefore, solely explored photocurrent integration approaches.
The proposed research can be implemented as an ASIC to minimize the system's size and noise further, enhancing its performance.
The silicon photomultiplier is a promising detector for bio-photonics measurement techniques, such as enabling auto-fluorescence to remove brain tumor margins.
Future research will examine the design's constraints in increasing the system's sensitivity to 1 pW-level incident optical power and enhancing and changing the dynamic range while preserving a linear response.
Georgel, R., Grygoryev, K., Sorensen, S., Lu, H., Andersson-Engels, S., Burke, R., & O'Hare, D. (2022). Silicon Photomultiplier—A High Dynamic Range, High Sensitivity Sensor for Bio-Photonics Applications. Biosensors. https://www.mdpi.com/2079-6374/12/10/793
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